According to two theoretical physicists, the identical discoveries that
sparked the search for a ninth planet may really be proof of a modified
version of gravity that was first created to explain how galaxies rotate
inside the solar system.
Researchers Katherine Brown, an associate professor of physics at Hamilton
College, and Harsh Mathur, a professor of physics at Case Western Reserve
University, made the claim after examining the impact the Milky Way galaxy
would have on objects outside of our solar system—should the laws of gravity
be governed by a theory called Modified Newtonian Dynamics, or MOND.
MOND suggests that the well-known rule of gravity by Isaac Newton is only
partially true. That is, MOND permits a new gravitational behavior to take
over when the gravitational acceleration predicted by Newton's law becomes
modest enough.
Some scientists view MOND as a viable alternative to "dark matter," the
term used by physicists to refer to a hypothetical kind of matter that would
have gravitational effects but not emit light, because of its empirical
effectiveness on galactic sizes.
"MOND is really good at explaining galactic-scale observations," Mathur
stated, "but I hadn't expected that it would have noticeable effects on the
outer solar system."
The Astronomical Journal
just published their study.
A'striking' position
Mathur and Brown have previously investigated the impact of MOND on
galactic dynamics. However, once astronomers revealed in 2016 that a few
objects in the outer solar system had orbital abnormalities that may be
explained by a ninth planet, they became interested in MOND's more local
impacts.
Prior to now, historic discoveries have been made due to orbital
peculiarities: Neptune was found by its gravitational pull on the orbits of
nearby objects; Mercury's minute precession served as early evidence for
Einstein's general theory of relativity; and most recently, astronomers have
inferred the existence of a supermassive black hole at the center of our
galaxy using orbital dynamics.
Brown noticed that the measurements that sparked the hunt for a ninth
planet might not line up with MOND's predictions. "We wanted to see if the
data that support the Planet Nine hypothesis would effectively rule out
MOND," she stated.
Rather, Mathur and Brown discovered that MOND accurately forecasts the
clustering that has been seen by astronomers. They suggest that certain
objects in the outer solar system would have their orbits pulled into
alignment with the galaxy's gravitational field over millions of
years.
"The alignment was striking," said Mathur, when they projected the orbits
of the particles from the Planet Nine dataset against the gravitational
field of the galaxy.
The authors issue a warning, noting that the available dataset is tiny and
that a variety of other explanations may turn out to be accurate. For
instance, some astronomers have suggested that observational bias is to
blame for the orbital irregularities.
"Regardless of the outcome," Brown stated, "this work highlights the
potential for the outer solar system to serve as a laboratory for testing
gravity and studying fundamental problems of physics."
Provided by
Case Western Reserve University
